Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and currently is of counsel at the New York law firm of Edelman & Edelman PC , focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan . The opinions expressed in this commentary are his own.

(CNN) Those waiting for shocking new revelations from James Comey in his congressional testimony next week are likely to be disappointed. After all, Comey is the same 6-foot-8-inch former FBI Director who attempted to "blend in" with the blue drapes during a previous White House visit with his then-boss, President Donald Trump, who fired him last month.

Spectators at the Senate hearing will probably be treated to a gossipy inside look at the quirky interactions between these two unpredictable men. Indeed, if Comey wishes to maintain any semblance of the reputation for integrity he so scrupulously cultivated before he blew up the presidential election, he will decline to reveal things that would compromise special counsel Robert Mueller's probe into contacts between Trump campaign aides and Russia.

Comey will also be boxed into a tight corner by his previous proclamations that he can't answer some questions because of national security and the need to maintain the integrity of ongoing investigations.

Additionally, he may be impeded in answering questions about the conversations everyone really wants to know about: those that Comey had with the President. Why? The President may attempt to assert executive privilege -- that is, he may claim that a private conversation in the White House with the director of the FBI -- a federal employee answerable to the President -- deserves confidentiality and full protection from public disclosure.

Should the President assert this privilege, Congress and Comey might argue that the doctrine doesn't apply here if the discussion was related to a criminal attempt to obstruct justice. In the end, the complexity of this issue and the need to possibly resolve the matter in court could bring the congressional questioning of Comey to a screeching halt.

As Comey's prior testimony indicates a desire to abide by the law, it is unlikely that he would ride roughshod over a claim of executive privilege by his former boss, the President of the United States.

During testimony given by then-Director Comey during the March 20 House Intelligence Committee hearing and his subsequent appearance before the Senate Judiciary Committee on May 3, Comey's most juicy offering was that that the FBI Intelligence Committee had clearly ascertained that the Russians and their leader Vladimir Putin hated Hillary Clinton and were actively attempting to assist in the election of Donald Trump. Let's recall that key exchange with Republican Rep. Mike Conaway of Texas:

Comey: Well, the assessment of the intelligence committee was, as the summer went on and the polls appeared to show that Secretary Clinton was gonna win, the Russians sort of gave up and simply focused on trying to undermine her.....

Conaway: Sir, do you believe that the FBI was consistent through early December and on that that was the case. That they -- they assessed that they really wanted Trump to win it and were working to help him win and her lose.

Comey: Yes, our analysts had a view that I don't believe changed, from late fall through to the report on January 6 that it had those three elements.

Enough for a headline, but not much more.

JUST WATCHED Clinton blames Comey, DNC for election loss Replay More Videos ... MUST WATCH Clinton blames Comey, DNC for election loss 01:15

On the issue of whether operatives of the Trump campaign or the President himself actively colluded with the Russians in their efforts to sabotage the Clinton campaign in hopes of electing Trump, Comey declined to answer questions, taking "no comment" refuge in his "pending investigation" mantra.

If questioned again in Congress next week about whether the President tried to obstruct the investigation of Trump campaign operative Gen. Michael Flynn, it will be impossible for Comey to walk away from his previous and oft repeated refusals to comment on specific individuals under investigation without shattering what remains of his own reputation for integrity.

Here are some samples of Comey's persistent refusals to answer specific investigation-related inquiries during the spring congressional hearings and questions from Democratic Rep. Terri Sewell of Alabama:

Sewell: Now, Director Comey, all of these accounts are open-source press reportings. Given Russia's long-standing desire to cultivate relations with influential US persons, isn't the American public right to be concerned about Mr. -- Mr. Flynn's conduct, his failure to disclose that contact with the Russian ambassador, his attempts to cover it up and what looks like the White House's attempts to sweep this under the rug?

Don't we, as the American people, deserve the right to know and shouldn't our FBI investigate such claims?

Comey: I can't comment. I -- I understand people's curiosity about our work and intense interest in it, and as Mr. King said, oftentimes, speculation about it. But we can't do it well or fairly to the people we investigate if we talk about it. So I can't comment.

And later...

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Sewell: What level of proof do we need in order for us to have a criminal investigation by the FBI of Mr. Flynn?

Comey: I can't comment on that.

Sewell: Shouldn't the American people be concerned what -- I think that it's really hard for us to fathom that he wouldn't know that he should've disclosed that he received $30,000 as a part of -- of a speaking engagement to RT, the Russian US anti-propaganda outlet.

Comey: I can't comment on that Ms. Sewell.

Under similar questioning by Sens. Al Franken and Richard Blumenthal and others about Flynn and other Trump campaign operatives, Comey refused to comment on the pending FBI investigations. A series of questions from Blumenthal are particularly revealing of how trapped Comey will be in his own "No Comment Zone" when questioned in future congressional hearings:

Blumenthal: With respect to the investigation, I'm going into the Trump associates ties to the Russian meddling. Has the White House cooperated?

Comey: With the investigation?

Blumenthal: Correct.

Comey: That's not something I'm going to comment on.

Blumenthal: Have you had any requests for immunity from anyone, potentially a target of that investigation?

Comey: I have to give you the same answer, senator.

Blumenthal: Would you tell this committee if there is a lack of cooperation on the part of the White House?

Comey: I won't commit to that.

JUST WATCHED Graham rips Comey for misleading Congress Replay More Videos ... MUST WATCH Graham rips Comey for misleading Congress 02:53

The history-making, big-bang revelations in congressional investigations often come from surprise witnesses rather than from the usual suspects. For instance, in the Nixon Watergate probe the existence of Nixon's White House taping system was revealed in a surprise tidbit of testimony from a little-known presidential aide named Alexander Butterfield. Nothing of similar magnitude came from White House big guns like Chief of Staff HR Haldeman or White House Domestic Affairs Adviser John Ehrlichman.

Similarly, though a repeat appearance by Comey will generate an enormous television audience, don't expect disclosure of important new evidence supporting claims of obstruction of justice against the President.

Instead, we are likely to hear that Comey is a careful note-taker, though it is doubtful that the actual notes will be produced. It is far more likely that Mueller will want the notes of Comey's presidential conversations to remain confidential until a sitting grand jury gets a look at them.

Comey is likely to cooperate with the request of Mueller, his respected law enforcement colleague and friend, and reveal little of substance to Congress. After all, Comey has a reputation to defend and even though now a civilian, he will feel bound by the FBI and Department of Justice rules of policy and ethics as well as the attorneys' Code of Professional Responsibility.

These rules strongly limit prosecutorial comment on pending criminal investigations. Comey lost his job and may well have influenced the American presidential election the last time he ignored these rules with headline grabbing revelations about the Hillary Clinton email probe.

Given that powerful lesson, it is unlikely he will comment on a pending investigation again even in his new civilian role. In the end, we can expect a cascade of colorful sparks and thick smoke but no real fire at the next edition of "Comey under oath".

Unless, of course, a new Alexander Butterfield emerges from the chaos of the Trump White House.