The cello is tuned an octave lower than the viola. While it is not uncommon for the cello to play in both the alto clef and even the treble clef, I suggest for ensemble passages to stay within the practical range.
The cello is known is used for its rich tone quality and has been used extensively in film music, notably the music of Hans Zimmer. Slow accented quarter notes in the lower register can add a powerful drive to a passage while a lyrical passage with cellos and violas can sing (Beethoven's 7th, 2nd mov't).
Arpeggios are remarkably effective on the cello, similar to how the left hand of the piano can be played. Many instruments can double with the cello, such as the basson, muted trombone, and euphonium.