NORRISTOWN, Pa. — Chelan Lasha was 17 in 1986 when she went to Bill Cosby’s suite at the Las Vegas Hilton, after, she said, he told her he could help with her modeling career.

She wet her hair to pose for modeling shots at Mr. Cosby’s request. Then he offered her what he said was an antihistamine to help her cold, and some almond liqueur. She took both, Ms. Lasha told a courtroom on Wednesday, because “I trusted him.”

Then, she said: “He laid me on the bed; I could not move any more after that. He kept pinching my breast and humping my leg. Waking up, I was naked.”

She said she heard him grunting, and with that, she imitated from the stand the sounds she said Mr. Cosby had made.