I've been playing a lot of diatonic everything. So basically just staying in the scale as I improvise with melody/chords. I play alot of songs by ear and I notice when they use accidentals but I don't really know the theory of how they do it besides that it just sounds good.
So I'm thinking it's just "whatever sounds good" and go with that. Or is there some theory in regards to how to add non-diatonic notes?
I want to play/improvise like this guy.
Using bebop scales is a great option because they contain chromatic passing notes, which is the sound you're looking for. There are many different bebop scales:
By adding a chromatic passing tone, these bebop scales gain something valuable: the downbeats emphasize chord tones. For example, looking at the C major bebop scale, the notes that occur on downbeats are C-E-G-A. These notes spell out a CM6 chord. The downbeats of the C dominant bebop scale spell out C-E-G-B♭, a C7 chord. The C melodic minor bebop scale spells out C-E♭-G-A, a Cm6 chord. And the C harmonic minor bebop scale spells out C-E♭-G-B♭, a Cm7 chord.
These scales teach us that it's important to think about where we want to introduce non-scale tones.
Another way to introduce non-scale tones is to approach notes chromatically. Chord tones, for example, can be approached from a half step below or from a half step above. Between the two, it's more common to approach a chord tone from a half step below. A common improvisational exercise is to surround chord tones. In the example below, I'm surrounding each chord tone of CM6 using a half step below and a whole step above:
The video you shared doesn't contain many chromatic scale tones (only one that I heard). What you're hearing in that video is related--chromatic passing tones. The video also contains more complex progressions that, while non-diatonic, are still very standard.
The progressions you hear in the video almost certainly aren't being improvised. But to develop an ability to improvise chord progressions, you need to learn songs and build up your vocabulary of chord progressions. Start out by transcribing the chords to the song you linked to. Find other jazz songs with similar progressions, and take note of where they differ. As you expand your vocabulary of known progressions, you'll develop a variety of choices you might decide to use on-the-spot while improvising a progression.
Some of the non-diatonic progressions you'll hear in that song alone include:
| IV Maj | ♯iv dim | I64 | V7 | I Maj || IV Maj | ♯iv dim | V7 || V7 | ♯v dim | vi min || II7 | V7 | I Maj | (note the II chord is dom7 rather than min7)| I Maj | ♭iii dim | ii min | V7 || ii min | iv min6 | I64 | V7 | I Maj |The best way to practice these is in context. Transcribe interesting chord progressions that you hear, play them on your instrument, try modifying them, and try combining different progressions. Categorize their function ("this one is a nice turn-around," etc.).
If you have a good understanding of diatonic systems, start thinking about the relationship between the notes outside of the scale and those inside the scale. As a baseline, it's important to understand that there is a relationship between all arbitrary collections of notes, diatonic or not. It's also helpful to know that for a given major key, seven notes are in the scale and only five are out, so you're already more than halfway there.
Let's say you're playing a tune with the chords C | Ami7 | Dmi7 | G7 |. Just before the transition between C and Ami7, someone plays a G#. How can you explain this note using an understanding of diatonic harmony? Well, G# is the leading tone of A minor, and its presence in the melody implies an E7 (or a G#o). Playing a G# effectively allows the soloist to imply a V7-i cadence going to that A minor. Because it also implies a G#o, you can also make a connection between any G# (... or B D F) diminished scale runs that you happen to hear in a C major system. You might also hear someone play the whole tone scale starting at C, going to G# and then landing on an A; you can explain the accidentals here (F# G#) by connecting those notes to E7-A. If someone played a Bb on this transition, you might rationalize it by noting that Bb7 is the tritone substitution of E7.
This is just an example and you can use that kind of analysis to explain the relationship between just about any set of notes. Indeed, you can certainly make more farfetched connections, but it's going to take more work for listeners to really grasp what you're doing. If you just start playing the Bb altered scale over that C major chord, you probably aren't going to be able to sell anyone on that connection. But you could get some pretty outside notes to sound "in" by properly contextualizing them; a fairly common example is people playing minor blues licks over tonic major chords, allowing you to get b3s and b5s to make sense in a major system.
I should reemphasize that this is kind of a wasted endeavor if your understanding of diatonic systems isn't really good. If it took you any significant amount of time to figure out why G# implies V7-of-vi in the key of C then your time is far better spent focusing on what that means than it is trying to explain every single note you see in a solo. But you should know that there is an explanation for virtually everything.
As some comments state (mine in particular). Everything should be done by ear. However there are some guiding principles for adding chromatics (accidentals) to a solo. (dejavu, I feel like I answered almost the same question last week).
Leading tone principle. When chromatics are added it is sometimes in leading up to a scale tone. One case in point is the harmonic or melodic minor. These have "accidentals" relative to the original key to produce a leading tone to the first note of the minor scale. This can be done anywhere.
Other scales. Two popular scales that have chromatic passing tones in them are the blues scale and the bebop scale. The true blues scale is minor pentatonic with a flat 5 added. The Bebop Ionian scale is major with a sharp 5 (flat 6) added.
Once you get used to the sound and feel using them in solos is easy. I think this is a good starting point. You can also make Bebop version of the melodic minor scale which sounds very cool.
READ lots of music. Don't worry about improvising for now, just play 'what the guy wrote'. You'll discover all kinds of possibilities. You can hang labels on them if you like. But I'd rather you spent the time playing even MORE unfamiliar music.
Look up secondary dominants and borrowed chords and modal mixture Here a some good videos on those things. Signals studio music is a great Youtube channel if you want you can check out some more of his video it would be worth it.
| ii min | iv min6 | I64 | V7 | I Maj |. You're latching on to those more complex progressions. The recording almost certainly isn't improvised, btw. – jdjazz Jun 20 '18 at 18:22