fiona shaw (born fiona mary wilson; 10 july 1958) is an irish actress and theatre and opera director. she is known for her roles as petunia dursley in the "harry potter" movie series (2001–2010), marnie stonebrook in the fourth season of the hbo series "true blood" (2011), and carolyn martens in the critically acclaimed bbc spy thriller series "killing eve" (2018–2022).