Overall the texture of Star Wars is generally homophonic. A better description of the texture might be melody dominated homophony because the melody - or theme - that is playing at any one time very much dominates the texture. Other melodies are less significant and take on a supportive role. This helps the tune to come across distinctly.
There are some melodies within the main section of the piece that could be described as countermelodies. An example is the inner part from bar 15 (0'33) that is played by brass. Though this ascending melody is very noticeable it is still subservient to the main theme.
There are some other interesting points to note about texture:
As is common in large orchestral arrangements, melodies are often heard in octaves. This is the case with the string melody heard from bar 12 (0'26) as well as the A section leitmotif heard from bar 21 (0'48). The doubling of parts on different orchestral instruments in unison also allows the melody to come across with more power.
The accompaniment in bar 9 (0'20) imitates the melody to provide a little textural variety.
Contrary motion. This is the opposite to moving in parallel (which is what happens with the brass at bar 11, 0'24). In bar 18 (0'40), the melody descends supported by a bass line that plays exactly the same rhythm but in an ascending pattern. This creates an effective and satisfying musical phrase which draws the music of the B section to a close very boldly.
Homorhythmic passages. Similar to the use of contrary motion, the music from bar 44 (1'41) is played by a number of instruments which all have the same rhythm. This rhythmic section is emphasised dramatically as a result.
Cast Away is a very emotional film about a man who has to survive on an island in the South Pacific while his wife and friends think he is dead following a plane crash. The music really helps support this and there are some noteworthy points relating to texture:
The piece begins monophonically with an oboe solo, which helps to conjure up a feeling of isolation and abandonment.
At 0'27, the strings play the melody with a chordal accompaniment. This homophonic texture helps the expressiveness of the melody to come across with a simple accompaniment that doesn't intrude or get in the way.