There is conflict. Decker and Jim are vying for control of the ship. Each suggestion from Decker breaks down into a fight. It isn't until Bones threatens Jim that he stops it. Spock is not present for part of this movie. They argue over what to do next "Jim, what the hell kind of plan is this?!?" "This is how I define unwarranted!" "Belay that Phaser Order!" It's pretty straight-forward in 12 Angry Men, too.
Yeah, I saw TMP. I don't recall much yelling and screaming between the crew members. I never said there wasn't conflict. What I said was that there "practically" isn't any
by comparison, and I even emphasized those words so that their effect wouldn't be lost. In other words, the interpersonal conflict in TMP pales by comparison with that in 12AM (hey, I'd never noticed
that before; 12AM, how apropos!)
A critique leveled at TMP is that Decker seems to hate Kirk in one scene, next we see them sharing a grim joke about how weak their hand is. Again, though, I say this a strength. Decker is a professional first and foremost--the mission means more to him than command*. Plus, he and Kirk have a history--Kirk recommended Decker for the Enterprise and, if we go beyond the screen, Will is Matt's son and Kirk was friends with Matt Decker, deeply respected him, suggesting that Kirk may have taken a similar interest in Will's career that he did in Garrovick's or Bailey's. The simmering, conflicting emotions of resentment and respect seems very real to me, made stronger by the fact that Decker only let's it show three times--and the last time he does it, he's not angry about losing the Enterprise. He's angry about losing Ilia--grief-stricken and yet he goes right back to doing his job.
Decker is perhaps the most Trek of Trek guest stars--he really belonged in that seat and we believe it.
*Contrast this with Riker and Jellico in "Chains of Command," Exhibit A in how poorly TNG handled conflict (although Boma in "The Galilieo Seven" is easily as bad--Boma wasn't the second lead) and why I can't take Riker seriously.