Though you may go on to make a career in portraiture, the first commission never comes again.
Savour the compliment.
Plan how to proceed.
For the client, having a portrait painted is a deeply personal undertaking.

• It's an emotional gamble: the painting will be viewed by family, friends and visitors to her home.
  If it's for a boardroom, it will be perused by her clients, peers and competitors.
  It may be reproduced in the news media.
  Depending on how skilful and intuitive an artist you are, it will be a revealing document.
  

For the artist, every portrait brings responsibility.
You need to know your own mind before you accept.
Your responsibility is serious and twofold.
1.
You must honour your own art.

• By this, I don't mean some nonsense about being excused from decent behaviour on grounds of 'artistic temperament.
  ' What I recommend is making sure the client knows how you work.
  For example, you may paint in Abstract Figurative style but he asks to be painted in photographic detail.
  

What are your options?

• You might respond with an outright refusal.
  Result: you lose the commission.
  If he feels insulted, he may bad-mouth you to his contacts, so you lose other prospective clients.
  
• You might feel like compromising, 'just this time,' in hopes of starting a career as a portraitist.
  Unless you're versatile in style, this painting is likely to fail.
  Result: you lose this client and gain a reputation as someone who can't deliver the goods.
  TIP: If this style problem arises, show your portfolio or draw attention to your paintings in the studio or gallery.
  

2.
You must honour the contract with your client.

• Your best option: have a careful discussion with the client at the outset.
  
• At this meeting, ask questions that help you understand what she wants.
  Give an overview of your method, such as such as how many sittings you expect to need.
  You'll have a good idea, from previous people-paintings you've done.
  
• Discuss the type of location and ask for measurements of the room where it's to hang.
  If you're with a gallery, its director will already have settled the matter of price.
  

What if you represent yourself at your studio?

• The task is easier when you have a schedule of the sizes you prefer and the prices at which they sell.
  Be sure to follow this meeting with a letter of appreciation.
  Include an outline of your method, terms and estimated time for delivery.
  TIP: add a few days to cover surprise delays.
  

What if the person is your friend or relative? What if her desire conflicts with your artistic ethic?

• Whatever happens, remember it's a business meeting.
  It's between you, the art-producer, and an art-buyer.
  Maintain a professional calm and courtesy throughout.
  
• Assure the person you appreciate the compliment and want to accept.
  Explain that to do your best work and do justice to his portrait, you need to work in your own, unique manner.
  If he persists, you can decline the job, respectfully and with regret.
  

Result: you might lose this commission.
There will be others.
You gain this person's respect.
You keep your self-respect.