Mera is not just a cameo in Justice League
http://www.morningnewsusa.com/justice-league-spoilers-amber-heard-as-mera-significant-for-plot-character-details-revealed-23101794.html
Great news! It looks like Amber's role is more significant than just a cameo in The Justice League. I'm happy about this because we have Wonder Woman, so we need to continue the tradition of having strong female heroines in the DC world. It looks like she is working really hard training 3 hours a day, I can't wait to see her in this role. I have been a fan since North Country and she has done nothing but impress me since.