Working with an ongoing client, I've seen the need first-hand to foster absolute openness in working relationships. Being able to share candid thoughts about the work we're doing together is important not only for making steady progress, but for building a foundation of trust on which everything else is based. Be comfortable with talking about things which might at first feel uncomfortable: money, your take on the work, your relationship with the rest of the team.
Begin by understanding, learning, and setting of expectations. Have those 'getting to know you' conversations amidst the torrent of work to discover how someone else operates and thinks: What makes them tick? What resonates with them? What are their concerns?
It's not always the case that someone can be prescriptive about what they need from you or what they expect (other than you just to 'get the job done' since that's why they're putting coin in your pocket) so responsibility lies with us to begin building the foundation.
Having this relationship from the perspective of an outsider may feel like a bias on my part. People paying for my time as a consultant may expect a certain level of candour; I am not a "yes man" by any stretch. However, I believe it's something which we should carry through any professional engagement.
You can be friendly; you can speak to their sensitivities; you can be mindful of what might offend them; you can be the shoulder when they need to vent. You can speak in a way which is kind and understanding, but also authoritative and knowledgable.
Façades are really easy to spot and people can tell when you're phoning it in or just saying what they want to hear.
To foster any open relationship you must speak honestly, speak openly, and speak with purpose. Speak with integrity. Speak to what you believe. Speak with your heart.
Each time you start a conversation with someone new, ask yourself: what can I do to make our communication open, honest, and purposeful?