I would suggest that you write the doll as if she has dialog like a normal character and have her "talk" while you are writing. I call this the "Groot Script" after a leak from Avengers: Infinity War directors reveal that in any Marvel Cinimatic Universe film that features the character of Groot, who speaks a language where the only dialog is "I am Groot" (though there is a fourth word)) there is a secret copy of the script that is given to Vin Diesel only that takes all his "I am Groots" and "translates" them into English so Diesel can say the 3 words with the right tone and inflection to convey what he is really saying in that line, and the manner in which to say it, and the response or set up his statement flows to and from.
This was also done by Peter Mayhew in his role as Chewbacca as well as limited use for R2-D2 and Darth Vader's actors. In all cases, the character's voices were dubbed in during post-production, so the suit actors (numerous R2 pilots, David Prowse for Vader respectively) would have scripts and deliver lines on set only for them to be dubbed after the footage had been shot. In the case of Chewie and R2, this had the bonus of allowing Harrison Ford and Anthony Daniels to reply as if the line was delivered by someone not howling or beeping and to keep on the proper cues. David Prowse's acting also served this for Mark Hamil but it's more famously known for allowing a major twist to be kept from leaking in the sequel "Empire Strikes Back" (and if you don't know it by now, shame on you). David Prowse was given the line "No Obi-Wan killed your Father" and was bewildered why Mark Hamil was over acting his disbelieving response (Hamil was only told the real line he had to respond to moments before the first take) and was shocked to learn that James Earl Jones (Vader's voice actor) delivered a completely different and more iconic line and Hamil was in the know (in fact, prior to the premire, only four people in the world knew the secret... Hamil, Jones, George Lucas, and Director Irvin Kershner (though Prowse thinks two others knew)).
The dialog for the "doll" serves the purpose of allowing you to script an actual conversation as well as pay attention to how the scene is staged and if the dialog stands on its own as allowing the reader to "fill in the blank" of the character they can't hear. Recall a lot of Han and C3P0's response to their vocally different partners are enough to understand basically what was said without knowing just what was said.
Another element to consider is having the doll posed, staged, or lit in the scene in a way that the way the girl and audience sees it fits the dialog... When planning out your dialog in your head, get a real doll and try to put it into poses and angles that will make the doll look as if her mood fits the conversation. V for Vendetta is a good film to get an idea as the titular hero is a man who wears a rigid Guy Fawkes mask the whole film and never removes it. His introductory scene is a good watch as through the actor's movements, the lighting, and the camera angles, V's mask changes its "mood" rather well for an object that is not flexible at all. He goes from dramatic to creepy to broody, to giddy, to charming and his "face" never looks out of place.