In Frozen,we see that Anna has habits of tucking her hair back and biting her lip.These habits are common when people are anxious or nervous;however,they are also often done by natural instinct,and lip-biting is commonly done when people are thinking hard and/or getting an idea.Despite being far more chipper than Elsa,Anna still has moments of being nervous in which these two habits come into play.

Even though Anna is optimistic,she is shown to lack confidence in and think very little of herself.Consider the fact that she grew up apart from Elsa for years,and never knew why. She thought that Elsa didn’t love her.Yet after all this time,Anna still wants and needs someone to love her.

So almost any time she is seen tucking a strand of hair behind her ear,it’s when she’s with another person.A person who could love her.Anna probably thinks that fixing her hair would make her look more clean and sophisticated.If she looks presentable to someone,maybe there is a chance that that person would love her.
For instance:
-When she talks to Elsa,she tucks her hair back because she doesn’t want her sister to reject her;
-When she is with Hans,she does it so she looks presentable to him,because he could love her;
-When she is with Kristoff,she does it because she’s been awkwardly staring at him a little too long and she doesn’t want him to think she’s weird.

Similarly,when Anna bites her lip, she does it a few times while in the presence of someone else,but she does it most when she is around Kristoff.
For instance:
-She does it when he places his desired items in Oaken’s shop on the counter,trying to act nonchalant about him being there;
-She does it when she starts to tell him the story of what happened with Elsa,thinking about where to start;
-She does it after she touches his head and he cries out in pain,worrying that she hurt him more;
-She does it after excitedly telling Kristoff that his new sled is the latest model,glancing back and forth between him and the sled.

The first few times Anna bites her lip when around Kristoff,she is feeling awkward and nervous around him,but it’s because he is a big,gruff and rude man,not because she is feeling anything for him.Later,however,when she has begun to see him in a new light and starts to fall for him,it is from being nervous at first,but then excited later.

So in summary,Anna tucks her hair back and bites her lip usually when she is nervous and anxious,and it almost always ties in with her desire to be loved by someone.

I'm not going to lie,I also have a habit of doing things like this.I often bite my lips,my fingertips,and curl up my hair a little but that's when I'm bored,especially from school.It's a habit that I really hate and I want to stop,but I can't.I guess we can't control our inner desires sometimes.
As always,Smell ya' later!