There is a false dichotomy here. You don’t have to look at it as ‘dreams versus reality’. You can look at it as ‘big ambition backed up by realistic hard work’.
If someone says they want to be an actor, say, ‘Great! Wow! I’m so impressed that you want to work that hard! Good for you!’. Praise the ambition, praise the goal, praise the person, never infect them with any negativity, and salute them for being willing to work that hard.
The point is that some people, especially young people, aspire to a given role (actor, sports pro or whatever) because they think it’s going to be an easy life. It’s not your job to dampen enthusiasm, tell them they don’t have a chance, or relieve yourself all over their cornflakes. What you can do, constructively, is help that person communicate with themselves in an honest and accurate way, and develop an understanding of the hard work and effort involved. If a girl wants to be an actress, fine. Let her taste what acting classes are like. Let her see those books of speeches that actors learn for auditions, and tell her to learn a few. Tell her to get roles in any drama she can, even a school play or amateur drama.
Help her to see the reality. To get to be a big star, you need to be a small star first. To get to be a small star, you need to at least get a speaking role. To get a speaking role, you’re going to have to go to auditions and impress the casting director more than 1000 other people (all just as determined and ambitious as you), and show them an impressive resume that suggests you’re trained, experienced and ready.
Can I just inject a personal note here, given that someone made reference to dreams like wanting to run your own business? These days, I live a very nice life. I gave up my job ten years ago, wrote a book that did very well and now my website business brings in enough revenue to live on, even if I don’t do anything all day. I literally make money while I’m asleep, or on vacation, or watching TV. Having this life is easy, and very enjoyable. Getting to have this life was stunningly hard work. Think toil, sweat, stress, every problem and setback you can imagine and others you can’t, long days, longer nights, tears, frustration, more stress and serious hard grind. It took me 9 months to get my first website business up and running, and I swear if I hadn’t been working out and keeping pretty fit at the time, just the stress from this alone might have killed me. Writing a book isn’t fun. Working out my entire business from end to end, production, marketing, retailing, order fulfilment and so on, was a major pain. But I told myself I wanted to do it, and I could, and I did. Now I don’t have to work for anyone else or commute every day or attend boring management meetings or do anything I don’t want to. And the money just keeps rolling in…
So there’s nothing wrong with aiming high and having ambitions. But there’s usually a heck of a lot of work involved. So long as the ambitious person realises this, or can be brought to realise it, there’s no need to input any discouragement. Negativity is cancer on ambitions, striving and effort. Have nothing to do with it. Think positive, dream big and work extremely hard.