This comes in Act IV, scene i, when Lodovico comes from Venice. Desdemona is saying that she would like to see the "division" between Othello and Cassio mended. The exact line you are referring to, in context, is:
Is there division between thy lord and Cassio?
A most unhappy one; I would do much
To atone them, for the love I bear to Cassio.
Fire and brimstone!
What he does here would have to be decided by the actor portraying Othello, since Shakespeare doesn't exactly give us his action. But Desdemona does hint at his state of mind (and at how innocently she meant her statement about "loving" Cassio) with the question, "What, is he angry?"
There is, however, a very significant action a few lines later. Desdemona again speaks favorably of Cassio, which incites this exchange:
I am glad to see you mad.
How, sweet Othello?
Othello (striking her)
So, though it does not come immediately after Desdemona's comment about "loving" Cassio, Othello doesn't take more than a few more lines to work himself up to hitting her. And in front of Lodovico. So, the rift between Othello and Desdemona is growing larger and larger, thanks to Iago's poisoning of Othello's jealous mind.