A new biography about Jack Nicholson is being released and one of the main selling points is that it discusses the affair Meryl Streep had with jack Nicholson while they filmed Ironweed which is not about the ounce of weed that grew up without friends because he was made of metal. This was the second movie that Meryl and Jack made within two years and apparently it was a non-stop sex party between the two. Meryl had been married for nine years when they made the movie but if you read the blinds it is not the first or last time Meryl cheated on her husband while making a movie. Jack would never let a relationship with another woman get in the way of him having sex with someone else so it probably wasn't an issue for him that Meryl was married.
There had been rumors they had hooked up during Heartburn and the reason they both agreed to star in Ironweed is they wanted to spend more time with each other.