Williams, 63, died from an apparent suicide one week ago.
Ward, who became good friends with him while shooting What Dreams May Come, said the film's recent popularity was "interesting".
Based on a novel of the same name, it follows Williams' character, Chris Nielsen, in his journey through the afterlife - which begins when Chris wakes up in a heaven which he can control with his imagination.
"It's interesting that they have gone for that film," Ward said.
"I think it was interesting that it was one of Robin's more dramatic roles, where he doesn't rely on his humour. I'd like to think it was a breakthrough film in some respects."
Ward admitted the film had been "a lot softer emotionally" than he originally envisioned, and said that was part of working with a large budget for a big studio.
"Perhaps that helped make it appealing for its wide fan base, as it becomes ultimately a very optimistic film."
It was remarkable that Williams had such a gift for making people laugh, considering the unhappiness that plagued his own life, Ward said.
"You just knew it was there."
Interest in several of Williams' well-known films has surged since his death, including Dead Poets Society, Mrs Doubtfire, Good Morning Vietnam and Good Will Hunting.