Pulver signed up to play Irene in August and will appear in one of the new episodes.
Pulver told Digital Spy at the Crime Thriller Awards that there will be a spark between Sherlock (Benedict Cumberbatch) and Irene.
"[Executive producer] Steven Moffat's written an episode about Sherlock dealing with what love is, rather than Sherlock really knowing what being in love is," she hinted. "We're both very much playing a mind game with each other. We've met our matches with each other."
Asked if there could be romance between the pair, Pulver teased: "It's kind of deeper than romance. It's kind of infatuation."
Sherlock is due to return in early 2012.
> Read more Sherlock coverage
Watch our video interview with Lara Pulver at the Crime Thriller Awards below: