Dawson Casting: Word of God is that Agnes' birthday party at the start of the second movie is her sixth. In the same movie Margo meets her first boyfriend, so she is most likely 12. Edith is about midway between these two. All three are voiced by somewhat older actresses (several years older, in the case of Miranda Cosgrove).
According to the DVD extras, Julie Andrews was chosen as Mrs. Gru specifically because, of all well-known actresses, she was the least suitable person to play an evil old woman — so choosing her to do so invoked the Rule of Funny. (She refused the part at first, but was persuaded.)
And GOB Bluth gives himself away pretty easily as Mr. Perkins.
On the other hand, though, did you hear Kenneth as the Tourist Dad at the beginning? No, you did not.
CarlyShay/Megan Parker/Summer Hathaway as Margo. Cosgrove had her voice debut in a 2005 Disney movie Peter Cottontail, but this is her prominently known voice actress feature film.
Mexican moviegoers may notice that the Latin American Gru sounds just like Mike Wozowski with an accent. Or Dr. Chunga.
Spanish moviegoers can see how Florentino Fernández is the voice for both Gru and his mother and the beautiful Ana de Armas (El Internado) plays Margo. You can even notice the accent Ana uses (in an interview she stated how hard it was to say "cisnes" (The Swan Lake is translated as El lago de los cisnes) correctly, because she would normally pronounce the 'c' as an 's').
The man working the carnival game (Aww, somebody's got a frowny face!) is Phineas and Ferb's number-one fan, Irving. Once you hear his voice again, it's pretty easy to connect.
According to Word of God, the minions are genetically engineered corn kernels.
And yet, they are constantly referred to as tater tots.
The Other Marty: Al Pacino was cast as El Macho in the second film, but was replaced late in post-production with Benjamin Bratt. His name could still be seen on most of the trailers leading up to the movie's release.