The beloved British star, 65, died at a hospital in Coventry, England on Thursday (28Mar13) after suffering complications following heart surgery, and a slew of stars, including Daniel Radcliffe and Matt Lucas, have flocked to pay tribute.
Now MCGann has opened up about his memories of Griffiths, who played Uncle Monty in the cult British classic alongside MCGann's character Marwood.
In a post on his Twitter.com page, he writes, "A brilliant, funny, principled man, and a true gent. Led the way on Withnail & I. Wore his great skill lightly and his great heart out. Rip."
MCGann's fellow Withnail & I star Richard E. Grant shared his grief with followers early on Friday (29Mar13) when the news broke, and he returned to Twitter to reveal he was planning to toast his former co-star.
Referencing the movie's famous drug-taking scenes, he declared, "Here's the plan. Going to fire up a Camberwell firm young carrot (cannabis joint) in memory of my 'Uncle Monty' and raise a figurative glass of sherry to him."
Grant also reached out to Harry Potter star Radcliffe on the site, as both actors had Griffiths playing an older relative in their movies.
He writes, "Am so so sad that we have both lost an 'Uncle' with the death of Richard Griffiths. He was my 'Uncle Monty' in my 1st film."
Actor Tom Hiddleston has also paid tribute by quoting a famous line delivered by Griffiths' character in Alan Bennett's film/play The History Boys.
On his Twitter page, he declares, "'Pass the parcel. That's sometimes all you can do. Take it, feel it, and pass it on. Not for me, not for you, but for someone, somewhere one day. Pass it on, boys'. Which he did. Every time. Dearest Richard Griffiths, we will miss you."