As mentioned above, one of the challenges you'll encounter is how to handle a never-ending line of people wanting to talk with you - mostly unrelated to solution. This section can be a book onto itself, and is no different than what's experienced by any other HR professional - employee or contractor. The bottom line is that as a consultant, you continuously have to prove ROI on your invoices, which is a different kind of pressure experienced by an HR employee. And whether the HR binders say it or not, most of those long conversations don't result in ROI for your client. In these conversations with employees, you're usually a sounding board, or quasi counselor or a complaint department when none of those are your roles. You're an expensive resource and all your conversations should end or work towards solution.
The challenge lies in finding the right mix between empathy and cost for the company. Those conversations aren't free, especially if you find yourself cornered into lots of them. And the truth is that many HR folk simply don't like to be disliked.
If this has been a challenge for you as an HR consultant, consider being known as the friendly but solutions-oriented HR person. As soon as someone walks into your office, let them know that you have 10 minutes unless it's an urgent matter. Or begin with "Hi Sally, what can I help you with today? The key is to ask it with a pleasant voice with absolutely no edge in your tone. Avoid "how are you?" which is an open-ended question where the answer could last an hour. (and what column do you post that hour to on your timesheet? Empathy?)
Regardless of the topic of conversation, come back to "How can I help you?" or "How do we get to solution?". You'll quickly be known as that nice HR person, who's busy and is focused on solution.