The actress was photographed proudly displaying her growing baby bump in a clingy purple dress while in character on the Atlanta set of the movie, about the real-life murders of three Arkansas boy scouts.
She portrays one of the slain boy's mothers in the adaptation of journalist Mara Leveritt's 2002 book Devil's Knot: The True Story of the West Memphis Three.
Witherspoon, whose first child with husband Jim Toth is due in late summer or early autumn (12), recently revealed she plans to take a break after giving birth.
She told Illinois newspaper The Chicago Sun-Times, ""After I finish (Devil's Knot), I'm going to take a little time off and go have a baby!""
But her maternity leave won't be for very long - the Oscar winner is scheduled to begin shooting Men Are From Mars, Women Are From Venus in January (13).
The new arrival will be a sibling for her son and daughter with ex-husband Ryan Phillippe.