One significant difference we see with respect to Rosalind's behavior toward Orland at court vs. in the forest is the way she addresses him either formally or informally. Rosalind new as a child that her father very much loved Sir Rowland de Boys, Orlando's father, but never met either Sir Rowland or Orlando before her father's exile. Since the moment she tries to dissuade Orlando from fighting the court wrestler Charles in Act 1, Scene 2 is the very first time she has met or has spoken with Orlando, naturally her manner of addressing him is very formal. We especially see her formality in the phrase "young man" in her line, "Young man, have you challenged Charles the wrestler?" (I.ii.168-69). The phrase "young man" is a very formal, very generic third person address. In other words, she could have been using the phrase to address anyone who fit the description of a "young man," showing us just how distant, proper, and formal she is being.
In contrast, when Rosalind sees Orlando again in the forest after she has fallen in love with him, due to something Celia says earlier about verifying his worthiness of being loved, Rosalind decides she wants to test his love for her. To test him, she decides to keep pretending to be Ganymede and speak to him "like a saucy lackey," meaning sprightly, bold, rude male servant. Under the disguise of a sprightly, bold, rude male servant, she questions the strength of his love, even saying,
Then there is no true lover in the forest; else sighing every minute and groaning every hour would detect the lazy foot of Time as well as a clock. (III.ii.302-05)
Hence, in this scene, she is behaving much more informally and much less like she would be bred to behave as a lady. Acting like a saucy boy gives her the opportunity to inspire him to confess feelings about her as Rosalind that Orlando would have hesitated to tell a woman.