Marvel And DC Wonderfully Collab For A "Peacemaker" Scene

Thanks to James Gunn, Marvel and DC Comics have collaborated to shoot a scene from Peacemaker and a test screening of Guardians of the Galaxy Vol. 3.
Even if the expectations weren't the highest, Peacemaker (the spin-off series of The Suicide Squad) proved to be an incredible success, fully convincing critics and audiences and deserving a renewal for a second season. Notably, the season finale that aired tonight in the USA marked a surprising milestone: James Gunn managed to get Marvel and DC to collaborate for filming a cameo!
Specifically, a DC actor in particular has reached the set of Guardians of the Galaxy Vol. 3 with the equipment of Marvel Studios, which they accepted without any kind of problem.

Source: HBO MAX

From here, watch out for the spoilers, as to report Gunn's statements we must necessarily reveal the identity of this character.
Well, you're still here, obviously you don't worry about spoilers at all.
The cameo in the question is Ezra Miller (please keep in mind that he is not the only one), who shot a scene as the Flash - with the same costume seen in Justice League - on the set of Guardians of the Galaxy Vol. 3 collaborating with the crew of the Marvel Studios.
"I just found out through some common friends [Ezra] liked my movies a lot," Gunn explained in a recent interview with Variety. "And so I asked if he would come in and shoot this scene, and he kindly agreed to do that. Actually, Marvel shot Ezra for us, while we’re shooting Guardians Vol. 3."

Source: Marvel Studios

According to James Gunn, Marvel had to return the favor to DC, as the Peacemaker crew shot Nigerian actor Chukwudi Iwuji's screen test for Guardians of the Galaxy Vol. 3.
"Marvel owed DC, because the Peacemaker crew shot my screen test for Chukwudi for Guardians Vol. 3. And then we got Ezra with the Guardians Vol. 3 crew," the director added.