For example:

Allowing about an hour for the talk . (That's usually the maximum that people can take without losing concentration; unless the speaker is unusually skillful and uses a variety of media, visual aids, and techniques.)

• Introduction from chairperson or host (5 minutes maximum, usually less)
• Speech (45 minutes)
• Questions (15 minutes)

You may want to allow more time for questions but some people will be losing concentration.

Pros:

• Simple and doesn't take much imagination to organise or facilitate
• Gets quality information from a high-value speaker
• Allows for depth (suits a complex and important topic)
• Allows some interaction for attendees
• Gives contact and exposure to interesting figures and ideas so they can be explored
• A popular format (people know what to expect and don't feel exposed or embarrassed)

Cons:

• It can be long for some people's attention span; deadly with a dull speaker
• Some people don't learn best this way
• Where there is controversy, usually this highlights only one point of view; no balance
• Restricts audience participation (the people asking questions will be the most extravert, not necessarily the best)
• Can easily be disrupted or hijacked by people who want to create trouble; or even well-meaning people in the audience who keep the microphone and give their own little speech, annoying everyone.

Speaker's costs will need to be considered and reimbursed.