Going by reports of how lively Beltran is in real life, any alleged "woodenness" in his acting would therefore NOT be his normal state. IOW, he would be acting as the character was written and as he was directed to act.
A poster commented on the two versions of "Dune"; how the lines were almost identical but the results were vastly different. The poster assigned fault to an actor. But this says nothing about what the director is ordering on the set during filming, and what orders the director has been given by the studio/producer/etc.
I've read that, in Voyager's case, orders came down from above that the actors of human characters were told to act far less emotionally, so that the non-human characters would be different. I don't know if this is true, but think about it--volatile B'Elanna is sedate; Janeway is clearly emotional only at the extremes (making her appear a bit nuts); Tom/Chakotay/Harry are so downplayed, the actors are accused of not being able to act; Tuvok has an excuse; Neelix is transformed into a clown. Meanwhile, the Doctor and Seven get to chew scenery. I'm not blaming those two actors--why wouldn't they want the opportunity to act?--but if the others were directed to act like their characters are sedated, you can't blame them for doing so and likely being pretty unhappy about it.