Eddie, 52, plays Jack's grieving father Arwel, and he made quite an impact on viewers during episode two.
Taking to Twitter, one fan of the series wrote: "Binge watching #ThePact@eddiemarsan welsh accent is spot on I must say."
Another added: "I have to say- I’m enjoying #ThePact, and the central performances massively, but @eddiemarsan is pulling off a very good, authentic Welsh accent!"
A third tweeted: "Eddie Marsan’s Welsh accent in #ThePact is actually pretty good, fair play."
Meanwhile a fourth viewer tweeted: "@eddiemarsan your Welsh accent is spot on!!! Love it! #ThePact #bbc1 #bbcwales." Credit Best TV News.
The Pact is available in full on BBC iPlayer and airs on BBC One.
