Ledger, who was the father of Williams' 2-year-old daughter Matilda, died last week in New York. The cause of his death has not yet been determined.
The couple split up last summer.
Williams had finished her last scene in the film "Mammoth" in Sweden when she heard Ledger had died, People.com reported.
"Blue Valentine" was set to start shooting Feb. 25, but has since been pushed back indefinitely.
"We will hold off until she is ready," an insider told People.com. "The production is in the process of sorting it out in respect to her."
The film co-stars Ryan Gosling.
"We're hoping (Williams) will still come back and do the film and are happy to wait," the source said. "The film is tailor-made for those two, so, of course, we would want to wait. You can't get much better than them."