What other kind of chords can a guitarist play as an accompaniment for the keyboardist instead of playing the same progression,that is ensuring they don't get in each others way by playing a monotonous tone?
First, you should definitely follow the same chord progression. You can't play two different chords at the same time without clashing, so unless you want a very challenging sound, make sure you're playing the same chords.
What you do want to do is take advantage of the fact that the piano is playing the notes of the chords. Also be aware of whether the piano is playing the bass line or if another instrument is playing the bass line. If the piano is playing all of the notes of the chords, and/or the bass line is being played, then you can (and many times should) leave out notes in your guitar playing.
I find it very helpful on guitar to not worry about the root or bass notes of chords when playing with other musicians. Instead of having everyone playing the root or the same bass note, you can leave some space in the arrangement, where the bass part can actually stand out more when only one instrument is playing it.
When the chords are played fully by another instrument, I like to get more minimal on the guitar (and vice-versa - when the piano solos, I would play fuller chords to keep the background sound "complete"). I love playing only three or even two notes at a time. That makes it possible to come up with some very interesting patterns, since you can choose any two notes from each of the chords and find combinations of diads that form little melodic or contrapuntal lines on the guitar. You can also change what notes and voicings you're playing on the repeats, which is a really easy way to keep the music interesting and developing. For example, you might play triads lower on the neck for the first and second repeats of a progression, but then when the last time around comes before the end of the song, you might go higher up and play diads that make an interesting pattern. Like an uplifting song you might find diads that ascend through the progression, or a descending pattern for a sad song.
Finally, there are plenty of single-note techniques that you can break out to go along with other instruments. Alternating back and forth between two chord tones or playing arpeggios along with the chords is a great way to add interest and motion to a chord progression. You can also play things that are like trills (repeating two adjacent scale tones) but really slow, perhaps with a swing feel. Selecting the right two notes is very important for the "trills". Taking the root of the chord as the first degree, you can get away with an 8-7-8-7, 3-4-3-4, or 4-5-4-5 pattern in a lot of situations. One trick for picking adjacent non-chord tones to play is to pick a non-chord tone that is a chord tone for the next chord or previous chord. Again this can depend a lot on the mood of the piece, whether anticipation to push things forward or suspension to hold things back sounds more appropriate.
PS: Don't forget that silence can be golden. You don't always have to be playing. Sometimes the best thing for a song is to let part of even all of it be played on piano. There are other times when just guitar is the way to go (perhaps with piano playing a bass line, perhaps not). Keep your focus on making the music sound great, not merely your guitar playing.
I can think of one example where this worked. There's a song by the Indigo Girls where the video had subtitles showing that the two guitars were playing different chords. The musical effect is of course just a larger, more complex chord.
Depending on the context (it may work for some songs but fail for others), you could try playing chords which share some notes as the first instrument but with a choice few extra notes which are appropriate extensions. Eg. If the first instrument is playing C (C-E-G), then playing an Em (E-G-B) on top of it merely adds a B which changes the overall chord to a Cmaj7.
But it's probably simpler to ignore the second chord and pay attention just to the added notes and how they change the resulting, overall chord.
To start trying this, I suggest adding 7s and 9s. These are likely to sound ok in many kinds of song. How to add 11s and 13s is something I'm still learning.
Choose which "stem role" or idea you're playing. In a sensibly organized non-chaotic pop song, there cannot be too many independent ideas about rhythm, harmony and melody.
For rhythm, you can think of the whole band as a big drum set, with some of the "drums" also having a harmonic and/or melodic dimension. The functional roles are: (1) kick drum, (2) snare drum, (3) hi-hat or ride, (4) accent cymbals (e.g. crash, splash, china). And that's it. Every one of the comping instruments locks into or doubles the role of one of these virtual drums. Note that the drummer doesn't necessarily have to play all of the roles, or all of if. He can play just 1/4 hi-hat and kick+snare, and then the guitarist or bassist creates a 1/8 or 1/16 pulse, playing a complementary hi-hat role. Ghost hits on the snare can also be considered as part of the hi-hat role, if they're not too strong.
For harmony the roles are the notes of the chords. If you want, you can divide the harmony area to subroles like, (1) the lowest note i.e. bass note, (2) the third of the chord, if it's a minor or a major chord, (3) tension notes i.e. sixths, sevenths, ninths, augmented or diminished notes, etc. (4) the highest note, which is usually slightly more easily distinguishable than the notes somewhere between the lowest and highest note. Anyone who plays a pitched note on any instrument, contributes to the overall harmony, and the strength of the contribution depends on whether they play their notes on strong or weak beats. What is a strong or weak beat depends on the rhythmic aspect of everyone's playing, the "one big drum set" talked about above! :) The bassist usually takes care of the lowest note responsibility of the harmony. (But the bass instrumen also has a heavy contribution to the rhythm roles, because each attack/onset and muting of a loud bass note is a major rhythmic event.)
The melody role is the leading most prevalent timed "pitch progression" that a layman could sing or whistle. Melody is strongly related to both rhythm and harmony. Melody has a rhythm of its own that's understood or perceived in relation to the comping rhythm's pulse, and the melody works as part of the harmony, outlining possible harmonic changes even if played separately without accompaniment. In addition to the main melody, there can be complementary melodies or counter-melodies. For example a guitar riff, brass riffs or backing vocal "answers" could be considered as having complementary melody roles. The lead vocalist sings something, and then a brass section fills in the quiet parts left by the vocalist.
Now to the question: what is a guitarist to do, if the keyboard player takes up all the roles and there's nothing left to do without having to exactly replicate what the keyboard player is doing? This may seem like a tragicomical situation, but it's not as rare as one might think. Pianists and keyboardists have a big arsenal of heavy weapons, and if they're not responsible and/or skillful, they can use their weapons to bulldoze everyone else and at worst turn the whole landscape into a big mush.
It would be best if you and the keyboard player could coordinate with each other - and the rest of the players (if any) - and decide what is the common idea for all the roles. If you think of yourself as one big drum set, what rhythm pattern are you playing? Rock'n roll, disco or samba? If you aren't comfortable with talking about music (or dancing about architecture) with the keyboard player, you could also just see what she's doing and adjust accordingly.
To pick and choose things from the selection of roles, you could for example do something like the following:
- Rhythm: Hi-hat, Harmony: single note, any chord note. This way you're essentially a pitched hi-hat. This is actually very common in disco, soul, pop type of things.
- Rhythm: Kick and Snare, Harmony: lowest note. This means essentially doubling the bass guitar's part, or what could be a bass part. On "kick" you play, on "snare" you slap/mute.
- Rhythm: Snare, Harmony: third of the chord and fifth/seventh. You're playing twos and fours, or a backbeat.
- Rhythm: only at every chord change. Harmony: full chords. This is like playing pad sounds on the keyboard. You just have to play the same chords as the keyboard player.
- Melody you double the lead vocals.
- Assisting melody + harmonizing you double the lead vocals a third above, like a background singer would do.
- Counter melody: Think that you're a horn section. Fill in empty space left by the lead vocals with counter-melody lines. This requires some melodic improvisation skills.
- Counter melody using chord tones: Fill in empty space left by the lead vocals with e.g. the 1st note of the scale, played as octave double-stops, George Benson style.