Just like in the original Star Trek television series, Leonard "Bones" McCoy plays an important role as Kirk's confidante in The Motion Picture. He's defiant, down-to-earth, and a little rough around the edges. Talk about a man after Kirk's own heart.
In a beautiful touch, Bones is the only crew member brought aboard Enterprise unwillingly. He's drafted. Based on his stylish clothes, he seems to have been enjoying a baller Gulf Coast-based retirement, so we can't blame him for not wanting to get back to work.
As soon as Kirk conveys the gravity of the situation, however, Bones is all business, which is a testament to the good guy that lies beneath his gruff exterior.
He's also great at real-talking Kirk when the Captain is at his most irrational. Check out this shade about his need to be in charge of the Enterprise:
McCOY: It's an obsession—an obsession that can blind you to far more immediate and critical responsibilities. Your reaction to Decker is an example.
If anyone else talked to Kirk like this, he'd slap them halfway to Romulus. Coming from Bones, however, these words just might make an impact.
Bringing Us Down to Earth
In a similar way, Bones takes a wonderfully no-nonsense approach to V'Ger, providing an atheistic, materialist perspective to contrast with Spock's increasingly New Age-y views on the nature of consciousness. This is one of our favorite interactions:
SPOCK: What it requires of its god, Doctor, is the answer to its question, Is there nothing more?
McCOY: What more is there than the universe, Spock?
This is Bones at his very best. We don't always agree with him, but we always walk away looking at things with a different perspective.
Typically a more cynical one.