The purpose of trash talk is to get the opponent mentally wrong-footed (to mix a metaphor). Just be careful not to lose your own footing because you're concentrating too hard on your oratory gymnastics and not your actual gymnastics.

If you don't like the idea that talking is a free action, you could enforce dexterity penalties for combatants who insist on talking too much while they fight. (Not sure this is a good idea - it's just an idea for something different.)

Transcript

Luke: You don't know who you're dealing with.
[SFX]: wmmm...
[SFX]: wmmm...
[SFX]: wmmm...
Darth Vader: I know enough.
[SFX]: Whooom!!
[SFX]: Whooom
Luke: You murdered my mother!
Darth Vader: You'll have to be more specific.
Darth Vader: Many have perished at my hands.
[SFX]: Kzzrzzk!
[SFX]: Whooom!!
[SFX]: Kzzrzzk!
Luke: You're a tyrant and deserve to suffer before you die!
[SFX]: Whooom!!
Luke: That's why your son is dead to you!
{Vader swings and disarms Luke of his laser sword; Luke overbalances and falls down stairs}
[SFX]: fall...
Darth Vader: Don't make me angry. You wouldn't like me when I'm angry.
Luke: You had me at, "You wouldn't like me."