There are two versions of a tickling scene in Solaris novel and Solaris movie.
Tickling in “Solaris” (1972)
In the novel, Kris is on the bed with the Hari replica. He lifts her leg and touches the sole of one foot with his finger. The girl smiles and, giggling, asks him to stop.
In “Solaris” (movie 1972) Kris is on the bed with the Hari replica; he tries to grab the gun placed next to her, and so he touches the sole of her foot. Hari squirms and with the gang kicks off the gun, claiming that he tickles her.
Laughter reaction to tickling communicates to the protagonist that the laughing girl is real, not a dream