Speaking to CultBox on the fantasy drama’s set in Cardiff last month, McGrath teased: “You left Morgana at the end of Series 3 in not a very good way. The new series opens up a year later and you discover that… the year has been spent schooling Morgana in the dark arts and in magic.”
She continued: “Also in this year she has spent it plotting her revenge against Camelot, not only for taking her position – as she sees it, because she sees herself as the rightful ruler – but also for injuring and, she believes, causing the death of her sister.”
Series 4 is expected to launch with a double-bill on BBC One next month.
The star added: “…she’s got a new plot, Morgana always has a new plan, and it’s a good one! The last one was good too, but you know.”
Watch our intervew with Katie McGrath…