Elsa is known for being poised, proper, reserved and regal like a queen should be. Although she has always been somewhat more mature than Anna, she was quite playful as a child. Though her accident to and separation from Anna caused her to grow up to be much more serious, she occasionally had her moments of being ungraceful, unladylike, and even slightly mischievous and sarcastic. Some of the most notable examples include:
After she and Anna simultaneously smell the nearby chocolate and say it out loud, they both let out a giggle, and Elsa brings her hand up near her mouth.
Right afterwards, the Duke bows to Elsa and Anna, and both girls stifle a laugh when the Duke’s toupee falls forward. Elsa maintains a soft giggle which she hides behind her hand.
After she watches the Duke drag Anna out of her sight (having tricked him about Anna being a dancer), Elsa waves slightly as she somewhat slyly whispers, “Sorry.”
Watching the Duke make a fool out of himself is enough to make Elsa giggle, which we see her do again (and hides it behind her hand) when Anna looks back at her.
When Anna returns, Elsa giggles once more before she clears her throat.
After Anna refuses to have a private conversation with Elsa regarding her engagement, Elsa curtly says, “Fine” before she adds, “You can’t marry a man you just met.”
Having just had her feelings hurt, Elsa excuses herself from Anna and Hans, the latter of whom starts to speak before Elsa snaps, “No, you may not!”
Elsa has quite a few moments like these as she sings “Let It Go,” especially because she wears a sassy smile. These include when she releases her cape from its clasp, when she tosses her tiara away, when she pulls her down from its bun and into a braid, when she walks out onto the balcony with her arms raised, and when she sings the final line before waving her hand to close the doors after she walks inside.
Like I said here, Elsa rolls her eyes when Anna tries to tell her what has happened to Arendelle.
After she has made a pair of skates for Anna, as Anna tries to say that she doesn’t skate, Elsa smiles mischievously before she playfully pulls Anna out into the rink.