Fellner's public acting career began with her role on the TV movie, So-Called Friends (1997) in which she played the role of Glory Factor. Some of Fellner's multiple roles on television include her appearance on the cult comedy TV show, The Fast Show (2000), which gave her the opportunity to play various roles with Paul Whitehouse, Charlie Higson, Simon Day, and the rest of the cast, My Hero (2002), The Line of Beauty (2006), The Eustace Bros (2003), Hollyoaks (2006), where she was directed by her now-husband Adrian Vitoria, Casualty (2007–2009), and The Trip To Italy (2014) as Lucy, among others. Fellner has also appeared in many films including the role of Claire in Two Days, Nine Lives (2001] Boogie Woogie (2009) with Heather Graham and Gillian Anderson, Little Deaths (2010–2011) as Maria Joy/Elizabeth Gordon, and The Trip to Italy with Steve Coogan and Rob Brydon, and Two Jacks (2012) opposite Jack Huston and Sienna Miller Sienna Miller, among others. After the success of The Trip To Italy, which premiered at the Sundance Film Festival, Fellner then made a second movie with Michael Winterbottom called The Face of An Angel (2014), about the Amanda Knox trial, which should be released in the US in 2015.