McKellen co-wrote, co-produced and starred in titular role in the British drama film directed by Richard Loncraine.
"Richard III" was adapted from William Shakespeare's play of the same name.
The 76-year-old actor, who was speaking at a special screening of the film here last evening, said choosing the perfect cast was a challenge and quipped that in the process he had missed casting "The Theory of Everything" star.
The actor, best known for playing Gandalf in "The Lord Of The Rings", is in the city to promote the British Film Institute's 'Shakespeare Lives on Film' - a global programme in honour to mark the playwright's 400th death anniversary.
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"I think I am more proud of this piece of work than anything else I've done. May be because it still exists, there are others too like Macbeth which still exist. But Shakespeare works well on screen. You can do it in many ways. Without words, in Hindi or Japanese," he said.
McKellen, who plays Magneto in the "X-Men" movies, also spoke about how he had famously returned his 50,000 pounds fee to complete the filming of the final battle sequence in "Richard III".
MAMI Film Club has partnered with the British Council and British Film Institute to bring the programme to India.
Mumbai is the first stop on McKellens' global tour as the BFI's ambassador for 'Shakespeare Lives on Film'.