In the old days back in the UK, we defined an expert as "an ex is a has-been and a spurt is a drip under pressure" (drip being a derogatory term for nerd - if there is such a beast).

But seriously,
An expert is someone who knows their particular subject well enough to overcome major complications. What an expert is not, is someone who knows everything about everything.

A consultant, (for some unknown reason has become synonymous with expert - usually a technical person) knows enough to see the broader picture and offer direction. They would consult with experts before suggesting the right course of action to their clients.

To be an expert consultant, I think four areas of expertise are needed.

Interview technique - to know which questions to ask and how to ask them to get the information you need.
Research technique - to know where/who to go to for specific technical advice
Project management - to provide timelines, cost analysis and oversee the project from start to finish.
Salesmanship - to sell your suggested course of action to your client.

Consultants are not necessarily technical people but they should be experts in their field.