What about when there is one beat of action, then the dialogue. Does the dialogue come on the same line, or is it a new paragraph?
Suz, I think-- what will the reader think? If the reader is going to think that the action and the speech are from different people because of the paragraph break, I think it's important then to put in a quote tag so the reader doesn't even have two seconds of wondering who the heck is saying this.
But personally, I would rewrite it so that someone else has the first line of dialogue at the end of the beat. Many reasons beyond just "so that you don't have Action by Bill and Speech by Bill in adjacent but separate paragraphs which Alicia doesn't like." :)
Another reason is replicating the rhythm of real conversation. If there's a lag, if there's a silence, usually the other person says something, even just "Uh..." to fill the space. Now maybe you're thinking, "But the action is taking just a split second," but it's a paragraph for the reader. It's not going to feel like a split second. So having the other character leap in with some kind of interruption will "feel" right.
And letting the other speak is a way to make this passage more interactive, more of a real conversation, not just "speaker and audience" but two characters interacting, each with his/her own agenda. What the other says, how he/she says it, will reveal something. If she's impatient, drawing his attention back to the subject-- "You were saying?"-- then we know that she wants to know more, that she might even think that he's avoiding the subject. If she says something helpful, like "I can understand how you feel," then we know more about their relationship.
Dialogue is a conversation, and I like it to look, feel, and sound like there are two people here, each with a reason to be conversing, maybe each with a goal, and certainly each with an attitude-- impatient, helpful, deceptive, something.
So if you want a character to insert some action, think about why. Action is a bit of a punctuator to speech. Why would he want to hesitate? What is he about to say or not say that makes him pause if only for an instant? And how is she going to interpret that?
I know some writers will say, "I just want to break up the dialogue with some action, or to anchor the scene in some setting detail." Well, sure, but that's not (if you do it well) what the reader gets. What the reader gets-- if you've made her believe in these characters-- is that the guy was conversing and stopped for a bit, and the reader is going to interpret this as having meaning in some way. Go with that. That's great. Let it happen. Make it happen. That doesn't mean you need a good reason for him to pause! But if you do it right, if you make the pause for action a part of the conversation, if you make the other character react, the reader will come up with a good reason. :) And that's what you want, the reader to get in there and care enough to speculate.