Are violas always part of an orchestra? How are they typically used? Do they often play the same parts as the violins to create a richer texture?
I'm new to orchestration and have been wondering about this.
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A standard string section of an orchestra will have:
One very typical way to think of this is by way of comparison to a four-part SATB choir (sopranos, altos, tenors, and basses). In this scheme, the first violins will have the melody line, like the soprano part of a choir. The cellos will have the bass line (doubled an octave lower by the basses). This leaves the 2nd violins and violas to play the so-called "inner parts" -- the altos and tenors, respectively. The violas essentially fill the role of the tenors for the string section. If you're really interested, studying standard four-part harmonization will give you an idea of how these inner parts are constructed. There's various kinds of guidelines for voice leading, such as rules about which notes of a chord should be doubled, and so forth.
Of course, this isn't the only textural option, especially once other instruments come into play. Sometimes the viola reinforces the bass line with the cellos. Or you might find the cellos and basses dropping out, leaving the bass line to be played only on the viola (this technique, called a bassetto, was popular in the solo sections of some Baroque concerti). You might decide to give the viola the melody, with a counter melody in the higher violins, or the violins might drop out completely, leaving the inner voices to a divisi cello section, and the basses alone on the bass line. Or you might want the entire string section (except maybe the basses) playing a single melody line in multiple octaves simultaneously, lending a "big" sound that can be heard, even over a noisy brass section.
Whenever it comes to questions of orchestration, it's always a good idea to study what others have done, by reading through a score while listening. There are tons of scores available on IMSLP, and there are also plenty of youtube videos that include a scrolling score. Note that when reading the viola part, it will usually be written using the Alto Clef which places middle-C on the middle line; this can take a while to get used to reading. To get you started, here's the opening to Mozart's Eine Kleine Nachtmusik, which contains some excellent string writing. Try to pick out and follow along with the violas.
Outside of unisono passages where everybody plays the same (apart from the octave choice and possibly some fifth thrown in for tecture), violins and violas are almost never playing the same: it would be wasting the distinctive texture difference between those two instruments. For fast passages, violas do not really provide the same kind of response and/or clarity which may partly be caused by their comparatively undersized body (being a fifth below a violin would imply a size increase by a factor of 3:2 which is definitely less even though varying, and being an octave above a violincello would imply a size increase by a factor of 2 which is definitely more). A viol would tend to be proportioned more in line with the other bowed string instruments.
So there is a decidedly different sonority, character, and agility to a viola as compared to a violin and a good composer would make use of it for giving a composition more complexity and plasticity than a one-dimensional doubling of a violin part would provide.
While some simpler orchestrations do use doublings of instruments and/or singers, those are usually picked from different instrument groups.
The four types of strings in the orchestra spend most of their time in different parts of the overall register. If you think about how the clefs work it becomes apparent. Think about where middle C is in the various clefs of the string instruments
Now, as you can see the notation for the violin generally speakling is above middle C. The violin can actually only go as low as the G below middle C. So generally speaking everything above middle C.
The Viola, on the other hand, is notated in the Alt clef with Middle C being here.
Now you can see that Viola can go pretty high but more so than the Violin centres around Middle C. The lowest note being the C below middle C.
The cello uses the bass clef and in this clef middle, C is here.
Again the Cello can play quite high but it revolves much more on the octave below middle C. The lowest note being two octaves below middle C.
The double bass is notated in the same clef as the cello but is, in fact, a transposing instrument. The sound it produces is, in fact, a whole octave below the notes as notated.
The lowest note as notated is the below the staff on the bass clef but in fact what you hear when the double bass plays that note is a whole octave lower.
SO in closing there is nothing that prohibits the player from playing really high in what is considered the other instruments range but yes as a general rule of thumb the each have their own part in the various octaves in which they operate.