Robin Williams’s Mork & Mindy co-star has said he repeatedly grabbed her breasts and bottom and exposed himself to her on the set, a new book reveals, but she excused it as part of Williams’s playful personality.

“I had the grossest things done to me by him,” said Pam Dawber, who played Mindy. “And I never took offence. I mean I was flashed, humped, bumped, grabbed. I think he probably did it to a lot of people … but it was so much fun.”

‘I really loved Robin and Robin really loved me.’ Mork & Mindy, 1980. Photograph: SNAP/Rex/Shutterstock

Williams would also wrestle her, break wind on her and come on to the set totally naked. He also once “goosed” an elderly woman playing Mindy’s grandmother by poking her between the buttocks with a cane.

The revelations are contained in a new biography of Williams by New York Times journalist Dave Itzkoff, to be published in May, and were reproduced in the Daily Mail.

“If you put it on paper you would be appalled,” said Dawber, 66. “But somehow he had this guileless little thing that he would do – those sparkly eyes. He’d look at you, really playful, like a puppy, all of a sudden. And then he’d grab your tits and then run away. And somehow he could get away with it. It was the 70s, after all.”

“It was just Robin being Robin,” said Howard Storm, who directed Mork & Mindy. Often Williams groped Dawber because he was “bored”, he said: “He’d be doing a paragraph and in the middle of it he would just turn and grab her ass. Or grab a breast. And we’d start again. I’d say, ‘Robin, there’s nothing in the script that says you grab Pam’s ass.’ And he’d say: ‘Oh, OK.’’’

The show’s producer, Garry Marshall, also recalled: “He would take all his clothes off, he would be standing there totally naked and she was trying to act. His aim in life was to make Pam Dawber blush.”

Mork & Mindy ran from 1978 to 1982, and starred Williams as a curious alien who lodged with Dawber’s Mindy in Boulder, Colorado. The show was Williams’s springboard to a successful movie career. He went on to star in Hollywood hits including Good Morning Vietnam, Dead Poets’ Society, Mrs Doubtfire and Good Will Hunting – for the last of which he won the best supporting actor Oscar in 1997.

Williams killed himself in 2014, following a history of depression, drug addiction and mental health issues. He was misdiagnosed with Parkinson’s and it was found after his death that he had had Lewy body dementia.

Despite his on-set behaviour, Dawber characterised her relationship with Williams as sisterly and spoke of his “gigantic heart”. “I really loved Robin and Robin really loved me,” she said. “We just clicked.”